Both Interjet and Volaris have up to 70% off promo code sales valid for purchase through Monday, April 2. Travel is valid throughout 2018.
Interjet and Volaris both have a similar baggage policy as the major airlines, which includes a total of 22 lbs. in two carry-on bags on their lowest fares. Interjet provides a generous seat pitch up to 34", which is comparable to JetBlue. Volaris has a cramped seat pitch around 29", which is comparable to Spirit.
For the Interjet promo, you just need to book on Interjet.com and it will automatically apply the discount when searching. The discount is advertised for travel through May, but flights are actually being discounted for travel through October 2018. They also have a separate promo, which includes one free checked bag on certain routes for travel through May. Free checked bag promo expires April 1.
For the Volaris promo, you'll need to select the "Up to 70% off" promo or just enter promo code VUEH70 when booking on Volaris.com for travel through December 2018. Volaris also has a separate promo with "up to 50% off" for travel in 2019.
Many routes are only operated on limited days each week. Lowest fares are generally available for fall travel. Click on the fare details for specifics. Both of these promos expire at 11:59 pm CT on April 2, 2018.